1. Field of the Invention
This invention relates to zoom lenses suited to 35 mm cameras, video cameras, etc. and, more particularly, to rear-focusing zoom lenses which, though being compact, have a high zoom ratio and, despite the employment of the rear-focusing method, maintain good stability of optical performance throughout an extended focusing range.
2. Description of the Related Art
The zoom lenses have generally the focusing method of moving the frontmost lens unit. Since the amount of movement of this focusing lens unit for the same object distance is constant, independent of the focal length, this method has merit in that the structure of the operating mechanism becomes relatively simple.
To extend the focusing range to very close objects, however, there is the necessity of providing an increased diameter. Otherwise, the light beam would be vignetted. In recent years, use of an electric motor in moving the focusing lens unit, or auto-focus camera, has been getting into the stream. Because the frontmost lens unit is far heavier than any of the other lens units in the photographic lens, the use of this front most lens unit as the focusing lens leads to other demerits in the consumption of electric energy by the motor and even in the focusing speed.
In a counterview to this, to reduce the difficulties due to such problems, there are many previous proposals for moving a lens unit other than the front lens unit in focusing, that is, the so-called "rear focusing" method, as, for example, in Japanese Laid-Open Patent Applications Nos. Sho 59-33418, Sho 59-52215 and Sho 63-195618.
In the Patent Application No. Sho 59-33418, the image forming lens unit, which remains stationary during zooming, is made to move in part to effect focusing. Because the focusing sensitivity E (movement of a plane of sharp focus with respect to movement of the focusing lens) is constant in the entire zooming range, the focusing speed has to be increased particularly in the telephoto positions. Even in the compact aspect, no sufficient improvements have been attained.
The Patent Application No. Sho 59-52215 discloses a zoom lens composed of four lens units wherein the third lens unit, when counted from the front, is made to move toward the relay lens unit to effect focusing. In view of the fact that the total length left is relatively long, however, much desired reduction of the bulk and size seems to have not been achieved yet.
The Patent Application No. Sho 63-195618 discloses application of the rear-focusing zoom lens to standard systems including wide-angle coverages. Also, the number of movable lens elements in the lens unit movable for focusing is so large that if all these lens elements are moved by a single automatic focusing device, quick focusing is difficult to perform.
As the five-unit type of zoom lens, mention may be made of U.S. patent application Ser. No. 444,942 filed on Dec. 4, 1989 now U.S. Pat. No. 5,136,431.